Field of the Invention
The present invention generally relates to ophthalmologic imaging, and in particular it relates to an image generation apparatus which generates an image of a certain region of an eye and a method for processing such image.
Description of Related Art
Inspection of a fundus of an eye is widely performed for early diagnosis of lifestyle-related diseases that are causes of blindness. A scanning laser opthalmoscope (SLO) is an ophthalmic apparatus utilizing a confocal laser microscope principle. The SLO apparatus performs high-speed raster scan on a fundus of an eye to be inspected, by using laser which is measurement light, and obtains a fundus image of high resolution which is a plane image from intensity of return light.
In recent years, an adaptive optics SLO (AO-SLO) apparatus has been developed. The OA-SLO apparatus includes an adaptive optics system which measures aberration of an eye to be inspected, in real time, by using a wavefront sensor, and compensates the aberration of measurement light and return light which occurs in the eye to be inspected, by using a wavefront compensation device. This adaptive optics SLO apparatus is capable of obtaining a plane image of high transverse resolution (hereinafter referred to as an “AO-SLO image” where appropriate).
Furthermore, in recent years, a method for observing a nonconfocal image generated based on scattering light obtained by changing a diameter, a shape, or a position of a pinhole formed in front of a light receiving unit has been used. An example is disclosed by, Sulai, Dubra et al.; “Visualization of retinal vascular structure and perfusion with a nonconfocal adaptive optics scanning light opthalmoscope”, J. Opt. Soc. Am. A, Vol. 31, No. 3, pp. 569-579, 2014. It is reported that, in the nonconfocal image obtained by this method, a structure of a fundus of an eye to be inspected may be identified in a region which is not resolved by the adaptive optics SLO apparatus (refer to Scoles, Dubra et al.; “In vivo Imaging of Human Cone Photoreceptor Inner Segments”, IOVS, Vol. 55, No. 7, pp. 4244-4251, 2014). Accordingly, in recent years, there has been an increase of interest in apparatuses which simultaneously obtain a confocal image and a nonconfocal image.
Such an apparatus which simultaneously obtain a confocal image and a nonconfocal image performs 2D scanning using measurement light from a resonant scanner or a galvanoscanner. Furthermore, a control circuit of such a scanner, for example, generates a synchronizing signal for image generation in synchronization with the optical scanning of the scanner so as to generate an image from a detection signal detected by a photodetector. Then, a fundus image is formed by matching (synchronizing) the signal indicating an optical scanning position of the scanner and an electric sampling position of the photodetector.
However, due to fluctuation of frequency of the resonant scanner, time delay of an electric circuit system, or the like, an optical scanning position of the scanner obtained from the synchronizing signal tends to mismatch the electric sampling position of the photodetector. Furthermore, since the adaptive optics SLO has high resolution, a difference between the optical scanning position of the scanner and the electric sampling position of the photodetector considerably affects the adaptive optics SLO. Accordingly, a large positional shift occurs between images of consecutive scan lines which are sampled in accordance with the synchronizing signal, and accordingly degradation of image quality, such as distortion of an image, tends to occur.
To address these disadvantages, Japanese Patent Laid-Open No. 2000-39560 discloses a method for obtaining or controlling an accurate position of scanning of the scanner using a dedicated hardware configuration. Furthermore, Japanese Patent Laid-Open No. 2014-68703 discloses a method for capturing a chart image for image compensation before imaging, obtaining distortion of an image from the captured image in advance, and compensating the distortion by an image process.
As resolution of an AO-SLO image becomes higher, an angle of view becomes large, and a frame rate becomes higher, a positional shift between images of consecutive scan lines becomes more relevant even if a shift between the optical scanning position and the electric sampling position is considerably small. In this case, even if the hardware configuration for detecting a scanner position is employed, as disclosed in Japanese Patent Laid-Open No. 2000-39560, it becomes considerably more difficult to detect a small shift between the optical scanning position and the electric sampling position with high accuracy and appropriate responsivity.
Furthermore, as for the shift between the optical scanning position and the electric sampling position, an amount of the shift may be changed in each imaging since the shift is affected by temperature in the apparatus, instability of a power source, and the like. Therefore, as described in Japanese Patent Laid-Open No. 2014-68703, even if distortion compensation is performed by image processing by obtaining a chart image for image compensation before image capturing so that image distortion is obtained in advance, distortion occurs in a captured image again and degradation of image quality occurs. In particular, in the case of ophthalmic apparatuses, it is difficult to simultaneously capture a chart image for image compensation and a fundus image using measurement light from the single resonant scanner in terms of a configuration of the apparatus, and therefore, it is difficult to compensate entire distortion in an image using the method disclosed in Japanese Patent Laid-Open No. 2014-68703.
Specifically, in general techniques, it is difficult to obtain an image in which distortion thereof caused by characteristics of a scanner is compensated without using a special hardware configuration, a chart image for compensation, or the like.